Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Two types of devices that are currently in widespread use are catalytic converters and diesel particulate filters. Catalytic converters contain a catalyst, which is typically coated onto a monolithic structure in the converter. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Diesel particulate filters are wall flow filters which have honeycombed monolithic structures typically made from porous crystalline ceramic materials.
In the state of the art construction of these pollution control devices, each device typically has a metal casing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The ceramic monolith generally has very thin walls to provide a large amount of surface area so it is fragile and susceptible to breakage. It also has a coefficient of thermal expansion generally an order of magnitude less than the metal (usually stainless steel) casing in which it is contained. In order to avoid damage to the ceramic monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal casing, protective packing materials (for example, ceramic “mounting mats”) are typically packed between the ceramic monolith and the metal casing.
The process of placing or inserting the protective packing material includes processes such as wrapping a sheet material (that is, a mounting mat) around the monolith and inserting the wrapped monolith into the casing.
Typically, the materials used to form mounting mats include refractory ceramic fibers that provide properties such as high temperature durability, good handling, resiliency, flexibility, and strength. An intumescent material can also be used to enable the mounting mat to volumetrically expand at elevated temperatures. Such expansion helps to hold the monolith in place during use.
Often, mounting mats are formed by bonding together two or more layers of protective material. For example, the layers to be bonded can be intumescent layer to intumescent layer, intumescent layer to non-intumescent layer, or non-intumescent layer to non-intumescent layer, depending upon the performance characteristics desired. Historically, the layers have been bonded together using an organic adhesive, which is typically a pressure-sensitive adhesive or a hot melt adhesive film. In general, the process for bonding the layers using a hot melt adhesive has been carried out as follows. First, the organic adhesive film is positioned between the two mat layers. Next, the mat is heated to soften the organic adhesive and passed through a nip to force the layers together. The mat is then allowed to cool.